The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) demodulator of a burst signal transfer system using an OFDM modulating method and, more particularly, to an OFDM demodulator in which a processing delay of a synchronization circuit can be shortened.
An OFDM modulating method as a modulating method promising for a high-speed data transfer such as a high-speed wireless LAN has been being examined. A conventional OFDM demodulator for receiving an OFDM modulated signal will be described by referring to drawings.
FIG. 4 is a block diagram showing the configuration of a conventional OFDM demodulator. FIG. 5 is a diagram showing a format of an OFDM burst signal.
As shown in FIG. 5, at the head of each burst data 23, a preamble 21 for symbol synchronization and a preamble 22 for estimating a carrier frequency and a channel characteristic are provided.
In FIG. 4, an OFDM burst signal is received by an antenna 1. An orthogonal component detector 2 performs quasi-synchronous detection (QUASI-SYNC) on the received signal by using a local signal having a frequency which is very close to a carrier frequency to thereby obtain an analog complex baseband signal.
A/D converters 3 and 4 sample and quantize an analog complex baseband signal of an orthogonal component and an in-phase component outputted from the orthogonal component detector 2.
A synchronization circuit 10 receives a digital complex baseband signal sampled and quantized by the A/D converters 3 and 4 and performs a synchronizing operation.
FIG. 6 is a block diagram showing the synchronization circuit 10. In FIG. 6, a symbol timing estimating circuit 14 which receives the digital complex baseband signal establishes symbol synchronization by the digital complex baseband signal sampled and quantized which is outputted from the A/D converters 3 and 4 at the time of receiving the preamble 21 for symbol synchronization shown in FIG. 5.
The symbol timing estimating circuit 14 outputs a symbol timing to a carrier frequency estimating circuit 11 and a symbol synchronization processing circuit 13.
On receipt of the preamble 22 for estimating the carrier frequency and channel characteristic having repetitive patterns, the carrier frequency estimating circuit 11 detects phase rotation in the same patterns which periodically appears, estimates a carrier frequency error, and outputs a frequency error compensating signal to a complex multiplier 12. A specific configuration of a carrier frequency estimating circuit is described in, for example, F. Daffara and O. Adami, xe2x80x9cA new frequency detector for orthogonal multicarrier transmission techniquesxe2x80x9d Proc. of VTC"" 95, pp 804-809.
A delay circuit 15 receives the digital complex baseband signal, delays the signal by predetermined time (which is about time of the preamble 22 for estimating the carrier frequency and channel characteristic) and, supplies the resultant signal to the complex multiplier 12.
The complex multiplier 12 complex-multiplies the frequency error compensated signal by the output of the delay circuit 15, thereby compensating a frequency deviation of the carrier wave.
After establishing symbol synchronization with the output of the complex multiplier 12 by the symbol synchronization processing circuit 13, the preamble 22 for estimating the carrier frequency/channel characteristic is outputted from the synchronization circuit 10.
A fast Fourier transform (FFT) circuit 6 in FIG. 4 Fourier transforms an output signal of a synchronization circuit 5 and divides the OFDM modulated signal into signals of subcarriers.
On receipt of the preamble 22, a channel distortion estimating circuit 16 receives the signals of subcarriers from the FFT circuit 6 and estimates a channel characteristic H(xcfx89).
The preamble 22 for estimating the carrier frequency/channel characteristic is used to estimate the channel characteristic H(xcfx89) in the channel distortion estimating circuit 16.
As a result of estimation, the channel distortion estimating circuit 16 outputs a coefficient 1/H(xcfx89) for compensating channel distortion to a channel distortion compensating circuit 8.
The channel distortion compensating circuit 8 receives the signals of respective subcarriers and compensates the channel distortion by complex-multiplying the signal by the coefficient 1/H(xcfx89) for compensating the channel distortion.
FIG. 7 is a block diagram of the channel distortion estimating circuit 16. In the diagram, the preamble signal 22 for estimating the carrier frequency/channel characteristics is supplied to a complex multiplier 161 in the channel distortion estimating circuit 16. The inverse number of the pattern of the preamble signal 22 is stored in a reference signal storing circuit 163 and is supplied to the other input terminal of the complex multiplier 161 in the channel distortion estimating circuit 16. Both input signals are multiplied by each other in the complex multiplier 161 and an estimation result H(xcfx89) of the channel characteristic is obtained as an output of the complex multiplier 161. The output H(xcfx89) of the complex multiplier 161 is supplied to an inverse number circuit 162 and a coefficient 1/H(xcfx89) for compensating the channel distortion is computed. The coefficient 1/H(xcfx89) is multiplied by an output of the FFT 6 in the complex multiplier 8, thereby compensating the channel distortion.
A subcarrier demodulating circuit 9 receives the distortion compensating signal and demodulates it every subcarrier.
As described above, the conventional OFDM demodulator has the delay circuit 15 in the synchronization circuit 10 used for compensating the carrier frequency deviation with respect to the preamble for estimating the carrier frequency and channel characteristic.
According to the orthogonal frequency division multiplexing (OFDM) modulating system, data to be transmitted is divided into a plurality of low-speed subcarriers. The period of a symbol is therefore long (generally, about 4 xcexcsec) and delay time in the delay circuit 15 is integer times or the unit of the symbol. In this case, such long delay time in the synchronization circuit 10 denotes an increase in the synchronization processing time on the symbol unit basis.
There is consequently a problem of a low throughput of a whole OFDM communication system due to the delay time in the delay circuit in the synchronization circuit.
In order to solve the problems, according to the invention, there is provided an orthogonal frequency division multiplexing (OFDM) demodulator for demodulating an OFDM modulated signal, comprising: a quasi-synchronous detector for converting the OFDM modulated signal into a complex baseband signal of two components by quasi-synchronous detection; a synchronization circuit for receiving the complex baseband signal and establishing symbol synchronization; an FFT for Fourier transforming an output of the synchronization circuit into signals of respective subcarriers; a channel distortion estimating circuit for compensating a carrier frequency error on the basis of an output of the FFT and a frequency error compensation signal outputted from the synchronization circuit and estimating channel distortion; a channel distortion compensating circuit for compensating channel distortion in the output of the FFT on the basis of an output of the channel distortion estimating circuit; and a demodulating circuit for demodulating an output of the channel distortion compensating circuit every subcarrier.
Specifically, a frequency error compensation signal is detected by using a preamble signal for estimating carrier frequency/channel characteristic in a carrier frequency estimating circuit in a synchronization circuit having no delay circuit. The frequency error compensation signal is outputted to a complex multiplier in a propagation path distortion estimating circuit. After phase rotation caused by a frequency error is corrected by an output of the complex multiplier, the channel distortion is estimated and compensated. As a result, the carrier frequency deviation is not compensated in the synchronization at the front stage of the FFT but is compensated after the FFT.